The present invention relates to a method and distilling of crude water, especially distilling of demineralized crude water.
In accordance with a presently known distillation technique (which is referred to as thermocompression), crude water is preheated by one or more heat exchangers, preferably to the boiling point. Distillate vapors are generated in an evaporator which is connected to the heat exchangers, and such vapors are sucked in by a compressor which increases the pessure of vapors to insure effective utilization of their evaporation enthalpy. As a result of such processing, the temperature of vapors increases so that the thus heated vapors can be used in the evaporator as a heating medium. The resulting distillate is caused to pass through and is cooled in the heat exchangers to thereby heat the crude water. Such method is economical; however, it exhibits the serious drawback that distillate must pass through the compressor (which can be said to constitute or form port of a heat pump). The compressor includes rotary parts which must be sealed from the surrounding atmosphere. Nevertheless, the seals for the moving parts are likely to allow the distillate to come into contact with germs of a bacterial nature, lubricant and abrasion material. Moreover, droplets of water which pass through the compressor with distillate vapors are likely to and normally contaminate the distillate.
In accordance with a conventional technique (known as thermodrive), vapors are relieved of water droplets by centrifugal force. Evaporation takes place in a tube serving to produce a spinning effect which results in centrifugal segregation of water droplets from vapors. Reference may be had to German Offenlegungsschrift No. 2,219,650, especially to FIGS. 4 and 5 of this pulication. FIG. 2 of the Offenlegungsschrift shows that energy is recovered primarily by resorting to several evaporation stages. Crude water is preheated in several condensers which are connected inseries and undergoes partial evaporation in a first evaporator in which it is maintained at a predetermined pressure (e.g., at a superatmospheric pressure in the range of 3-7 bars). The non-evaporated portion is introduced into and is caused to partly evaporate, at a lower pressure, in aother evaporator. The remnant of crude water is caused to evaporate in the last stage. Distillate vapors of the first stage are used as a heating medium for the second stage, those of the second stage as a heating medium for the third stage, and so forth. Distillate vapors of the last stage are supplied to the first stage by way of a heat pump. If the evaporators are or include several centrifugal separators in the form of tubes which produce a spinning effect, the resulting distillate is purer than can be achieved by resorting solely to the aforediscussed thermocompression technique. However, the heat pump in the last stage is likely to pollute the distillate. In the absence of such heat pump, it is necessary to supply external energy which increases the cost of distillate.
Thus, the just described conventional distillation techniques cannot produce a distillate of satisfactory purity, except by resorting to centrifugal purification and multi-stage evaporation without a heat pump. In the absence of a heat pump, the distillate is much more expensive because it must be subjected to an additional heating action.